The reasons for their invention and immense popularity are many. A ready internet access exposes computers to various security hazards like Trojan horses, viruses and spyware. It is extremely important to guard the data against these threats. At times it is not possible to do that with the help of firewalls and anti virus system also.

Also, a lot of times, a computer has multiple users putting any sensitive information in the computer at great risk of corruption and loss. Apart from this, there is an increasing demand for storage space due to multimedia uses like MP3, Videos, photo files and so on. All these take up a lot of space on the hard drive. External hard drives were invented keeping these requirements in mind.

An external hard drive consists of a hard drive and its own casing or enclosure. It sits outside the main computer tower in this enclosure. The enclosure is slightly bigger than the hard drive, and at times contains a cooling fan. This portable encasement allows the user to store information on a hard drive that is not inside the computer.

The external hard drive is connected to the computer through a high-speed interface cable, usually with a plug and play interface. The interface cable allows the external hard drive to communicate with the computer to transfer the data back and forth. The most common types of interfaces are USB and Fire Wire.

A portable or external hard drive is quite a useful and has various advantages:

1. Safety from Internet Threats: It allows the user to back up or store important information which can be compromised by online or offline activities away from the main internal hard drive. Sensitive documents, large music files, DVD images, movies, and even backup of the contents on main internal hard drive, can be kept safely on an external hard drive.

2. Portability: It is portable and operates on a plug and play basis. Any computer with USB or Fire Wire capability can recognize the external hard drive as a storage device. The drive can be accessed like a normal internal hard drive. It is easy to transfer huge files from one location to other or between your desktop and laptop. All it needs is to plug in the interface cable to quickly reconstruct a working environment.

3. Security: If there are multiple users of your computer system, external hard drive can keep all your important and sensitive documents safe. You only need to can plug in the external drive and make all your data and programs available. When finished, simply unplug the drive and take it with you or keep it in another secure location.

4. Segregation of Information: You can buy several hard drives and swap them in and out of the same enclosure, using one for multimedia storage, one for imaging, one for backup, one for work, and so on.

5. Data Recovery: They are ideal medium for recoverig the data from a broken or damaged computer or internal hard drive when a separate hard drive is required to store the recovered data.

6. Strong Casing: The outer casing on the external hard drives as compared to their non existence in other portable media gives them better protection from damage, age, weathering, and corruption, than CDs, DVDs, and Flash Drives.

Though overall there are more advantages than disadvantages of using external hard drives they are not completely fool proof:

1. Corruption: External drives are subject to data loss risks that are similar though lesser in proportion as internal hard drives: viruses, worms, data corruption, etc. due to their direst access to the computer they bypass anti virus installation and firewalls.

2. Failure prone: They are notoriously failure prone due to human mishandling (scratched, cracked, bent, misplaced). External drives are easily damaged, lost or stolen due to their portability and small size.

3. Storage: Where can one store external hard drives -- a safety deposit box, a glove compartment, a purse? Its proper storage should be in a climate-controlled environment. They are vulnerable to degradation by the environment (oxygen, heat, sunlight, humidity, liquids, and dust).

Weighing the pros and cons against the specific requirement can help one decide whether to use an external hard drive or not.